The assessment of perceptual capacities generally requires measurement of the subject's ability to integrate information from more than one stimulus dimension. Judgments of this kind are susceptible to various biases arising from cognitive, motivational, and other context effects, often invalidating the measurement. A general methodology, known as functional measurement theory, is being developed to deal with this problem in behavioral information processing. A study was undertaken to evaluate the usefulness of the functional measurement approach in assessing perceptual constancy judgments. These judgments involve the integration of size and distance information and are known to be affected by attitudinal biases. The question to be investigated was whether the functional measurement procedure would result in valid measurement of perceptual functioning in a situation where specified artifacts and bonafide perceptual effects are known to occur.